An agricultural machine, used for tilling by linking a plow to a tractor which has a hydraulic device for lifting and lowering the working machine through a link mechanism, generally possesses (a) a draft control function for moving the plow up or down depending on an increase or decrease of traction resistance, and (b) a tilling depth setter for setting the tilling depth in a specified range. The draft control is intended to detect the load applied to the link by a draft sensor provided on the top of a link mechanism. The plow is lifted or lowered by the hydraulic device to keep this detected value within a preset range determined by the tilling depth setter, thereby adjusting the tilling depth.
When lowering the plow, it is effective to perform a so-called plowing-in control, that is, to fully open the hydraulic valve to suddenly lower the working machine, in order to bite into the soil favorably. However, when the tilling depth setting from the tilling depth setter is shallow, the plow was lifted above the farm field, depending on the type of plow, when the draft operation was started after the plowing-in control. In this case, if the plow was lifted above the ground, the operator often continued work without noticing this state.